1. Field of the Invention
This invention relates to semiconductor materials and, in particular, to the detection of defects in semiconductor materials.
2. Art Background
Due to its advantageous electrical and crystallographic properties, indium phosphide has been actively considered for use in a variety of semiconductor devices. These devices are contemplated for many diverse applications, e.g., detectors in optical communications and photovoltaics for solar energy collection. For example, it presently appears that optical communication systems will depend on transmission of light in the 0.9 to 1.6 .mu.m wavelength region. Indium phosphide is generally used as a substrate for or as a heterojunction with semiconductor materials such as Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y, having bandgaps appropriate for detection in the 0.93 to 1.65 .mu.m region. The use of InP is particularly advantageous since the bandgap and electron infinity of InP is suitable for forming rectifying junctions with many semiconductor materials used in applications such as photovoltaics or photodetection. Also, the lattice parameters of InP appropriately matches that of many useful semiconductors and leads to mechanically stable interfaces.
To fabricate a satisfactory device, the materials used to form it should be essentially defect free, i.e., defect densities less than 10.sup.4 cm.sup.-2. If materials having a significantly greater defect density are employed inadequate operation, e.g., poor photodetection of the device, is generally the result. In this regard, since it is less expensive to discard unsuitable semiconductor material than to discard unsuitable finished devices, defect measurement before fabrication of a device generally is performed.
The most common method of defect determination is the etch pit method. For InP, wafers that are used for device fabrication typically are cut from a boule produced by a suitable crystal growth method, e.g., liquid encapsulated Czochzalski (LEC) pulling. After the boule is formed, representative slices, e.g., from the top, middle and the bottom of the boule, are taken. These slices generally are cut perpendicular to the growth direction so that the surface of the wafer includes material formed essentially simultaneously. This surface usually is treated with an etchant that produces pits in the wafer surface at defect sites. The number of pits are considered a relative measure of the defect density.
Although numerous etchants for an etch pit determination have been tried for InP, a completely satisfactory etchant has not been found. For example, a 6:6:1 mixture of HCl, H.sub.2 O and HNO.sub.3 was employed to determine defects in InP (see G. B. Mullin et al, Journal of Crystal Growth, 13/14, p. 640, (1972)). Although this etchant was somewhat successful for use with undoped InP, etch pits were not obtained on LEC-pulled zinc-doped InP. This is particularly unfortunate since, presently, zinc doping is a frequently used method for obtaining p-type InP--the type generally used in photodetecting devices. Similarly, an etchant employing 10 ml of water, 40 mg of silver nitrate, 5 grams of CrO.sub.3, and 8 ml of HF also was not useful for the etch pit determination of defect densities in zinc doped InP. (See T. Iizuka, Journal of the Electrochemical Society, 118, p. 1190 (1971), and G. A. Rozgonyi and T. Iizuka Journal of the Electrochemical Society, 120, p. 673, (1973).)
One etchant described by A. Huber and N. T. Linn, Journal of Crystal Growth, 29, p. 80 (1975) did show etch pit formation on the (111)B face of zinc-doped InP. However, the results achieved with this etchant depended very strongly on the method of surface preparation. For this reason, poor reproducibility was obtained making the etchant inadequate. (See F. A. Thiel and R. L. Barns, Journal of the Electrochemical Society, 126, No. 7, pp. 1272-74 (1979).)
Attempts to adapt etchants used for the etch pit determination of defect densities on semiconductor materials related to InP have not been successful. For example, the use of a solution of 40 parts HCl, 80 parts nitric acid, and 1 part bromine, (see R. C. Clarke et al, Journal of Materials Science, 8, p. 1349 (1973)) although useful for etch pit determination in undoped GaP was not successful for the corresponding use with InP. Thus, known InP etchants for defect determination are deficient, and attempts to modify known etchants used in similar determinations on related semiconductor materials have not been availing.